Growth of Wollemi Pine in the wild

Most of the adult Wollemi Pine trees, and a number of seedlings and juvenile plants, have been tagged and measured for height and trunk diameter. This provides a basis for long-term monitoring survival and growth rates.

So far, tagging juveniles at one of the sites has shown that their survival is high, though their growth is very slow. This possibly suggests that juvenile plants need a break in the canopy before they can grow and mature into adult trees. However, a break in the canopy is likely to be rare, caused by tree death and fall following disturbances such as storms, rock falls, fire or floods. (See If trees could talk...)

The slow growth of young plants in the wild contrasts with their faster growing habit in a controlled environment like a glasshouse or cultivated plants in the ground. (See also Growth of the Wollemi Pine in Cultivation)

Seed production

Seed production varies from year to year in most plant species. Some species in the family Araucariaceae, to which the Wollemi Pine belongs, have a “masting” seed production habit. This is where enormous numbers of seeds are produced every few years, with very few seeds produced in intervening years. We wondered if Wollemi Pines were also a masting species. To investigate further we used the long-term photographic record of Wollemi Pines, taken from above Site 1. Zooming in on these photos, we were able to count individual cones on two individual trees, and develop a 12-year cone production record. We actually found that inter-annual variation in cone production (an indication of seed production) was low, relative to other species. There is no evidence that the Wollemi Pine is a mast seeder. We also found that cone production was related to temperatures and rainfall, indicating that changes to climate may affect seed production. Nevertheless, the impacts of any variation in seed production on the Wollemi Pine is likely to be buffered by the species’ juvenile bank – small trees that survive in the shade, year after year, with very little mortality.

Seedling growth in the wild

Seedlings are important because they are the next generation. For Wollemi Pines they are particularly important because they have the potential to grow into whole new Wollemi Pine individuals and increase the population. Wollemi Pine seedlings at Site 1 have been monitored since the site's discovery, with scientists following growth, survival and mortality of individual plants.

When we analysed the past 16 years of monitoring data we found that 65% of new seedlings did not survive for even one year – disappearing before they could be surveyed a second time. Alternatively, 44% of established juveniles (big seedlings) survived the 16 years of monitoring, and potentially longer, as they were present at the site before we started monitoring. Together these results indicate that while early seedling mortality is high, once established, juvenile Wollemi Pines have good survival rates. However, Wollemi Pine seedlings and juveniles grow incredibly slowly in the wild, less than 1 and 2 cm per year, respectively!

Slow growth is not necessarily a problem. Many rainforest species in the wild maintain a slow-growing juvenile bank – with small plants just waiting for the right opportunity to grow up, such as when an old tree falls over and light shines down to the forest floor through a gap in the canopy, providing increased resources and growing space for the next generation.

Wollemi Pines and fire

For a long time people have thought that rainforest is sensitive to fire; if a fire burnt a rainforest then all the plants would die and not recover. However, recent research has shown that many rainforest plants can survive fire and have the ability to recover after fire. Wollemi Pine is one such rainforest species. Although fire is listed as a key threat in the Wollemi Pine Recovery Plan, scientists noticed that some mature Wollemi Pines had fire scars – indicating they had survived fire. We experimentally burnt small Wollemi Pines, alongside other common rainforest species, Lilli Pilli and Sassafras at a range of temperatures. All three species re-sprouted, indicating they can survive and recover from fire.

In another experiment we also found that the leaf litter generated by Wollemi Pines is very flammable, especially compared to leaf litter from other rainforest species. We are not sure whether this higher flammability has evolved for a reason (e.g., to disadvantage more fire sensitive species) or by chance (i.e., because Wollemi Pine sheds whole branches, litter is more aerated, drier, and therefore more flammable).

Wollemi Pines and drought

The Wollemi Pine is found deep in the Blue Mountains, in a canyon with a creek running through it. Its habitat is cool and moist, compared to the surrounding environment, especially the sandstone ridges above. All of these things suggest that Wollemi Pines prefer wetter environments to dry ones.

Physiological testing of Wollemi Pine vulnerability to hydraulic dysfunction (e.g. embolism or air bubbles in the stem) in response to drought showed that Wollemi Pines were resistant to hydraulic dysfunction, when compared with other species from Araucariaceae. However, when we withdrew water from potted Wollemi Pines, to simulate drought, our results indicated the opposite was true: the leaves and branches of Wollemi Pines turned brown and died back faster than other species from Araucariaceae.

Why did drought cause Wollemi Pines to die back more quickly than other Araucariaceae species? Part of the reason may be that Wollemi Pine leaf water potentials declined, while the other Araucariaceae species held their leaf water potentials steady in response to drought – we’re not exactly sure why the Wollemi Pine does this.

It has also been suggested that the increasing aridity of Australia, millions of years ago, contributed to the contraction of Wollemi Pines, from a much larger distribution, into the small area we know them from now.

If Wollemi Pine really prefers wetter environments, why didn’t it contract to somewhere wetter than the Blue Mountains, like tropical north Queensland? We’re not sure, it might be related to Wollemi Pine’s preference for cooler temperature extremes, compared to hotter ones (Offord 2011). It could also be related to Wollemi Pine soil preferences, or soil microbial associations.

Wollemi Pines and temperature

Predicted climate change scenarios may impact future survival of wild and cultivated plants, by changing the timing and frequency of exposure to both higher and lower temperatures. In cultivation, the critical temperature threshold for Wollemi Pines may be reached or exceeded in many situations due to upward temperature shifts. Overall higher temperatures may contribute to insufficient cold hardening, which would usually help plants gradually adapt to lower temperatures and frosts. Predicting the response of the wild Wollemi Pine population to climate change is more complex: will their habitat warm more quickly than surrounding areas due to their elevation, and will this effect be mitigated by the cooling and sheltering effect of the deep gorges?